Release history
Nexus descended from Proximity, which was the first MRM available. It was updated architecturally to be lighter and faster using the lessons learned from Proximity. Nexus also has the most active release cycle with Major releases every 6 weeks and minor releases much more frequently.

Embedded Jetty. JSW launch scripts - runs as a service on windows and unix

Jetty - runs as a service on Windows and Unix. Complete script for installation as a service on unix, includes: user creation, permission settings and service config.
Complete script for installing a standalone Tomcat service on Unix.

Embedded Jetty. JSW launch scripts - runs as a service on windows and unix. Also Native Solaris Service Manager Scripts.

War

"Drop the War" - deploy into any servlet container with zero configuration.

Fully extensible across the core components, REST API and UI. See herefor more info.

Proxying and Cache

Hosted Repositories

Proxy Repositories

Aggregate Repositories into single logical repo

Nest and reuse Repository Groups

Inclusion/exclusion rules per remote proxy

Lookups via groups can be optimized using routing rules. Since most lookups are done via a group url, this is the most effective way to control and speedup lookups. Individual artifacts can be controlled via the repository targets, which can be set per repo or groups of repos. See the procurement plugin in Pro below for more full featured control of artifact inclusion/exclusion by definable rules

RSS Feeds and UI viewer for bad checksums and artifacts with bad poms.
Bad poms are allowed through by default because many times Maven can still use them. We don't believe that simply inserting a repo manager should cause things to suddenly fail from Central. The repo man should for the most part be transparent by default.

Repository Statistics

Per repository or as a comparison among multiple repositories

Number of stored artifacts. More stats coming.

Artifact Statistics

Download count.
Last downloaded and by whom.
Deployed by.
Age.

RSS Feeds for New Artifacts

RSS feeds available both for new artifacts in the repository and for newly deployed/discovered versions of a specific artifact

Which is intentional because Maven doesn't actually need the full WebDav protocol. Since Nexus handles the data on disk, the http PUT is all that is needed. The standard lightweight http wagon can be used for deployment. Most of the Java implementations for the server side are non-compliant. Nexus goes for simplicity and performance.

No Wagon Extension Required (works with lightweight-http)

Deploy Artifacts via UI

Includes snapshots and ability to auto-generate poms and edit pom before deployment.

can auto-generate poms.Accepts multiple files in one operation to accept classified/attached artifacts.

Manual deploying of SNAPSHOTs is not allowed as this is bad practice. 3rd party SNAPSHOTS should get converted to an internal release version so you can reliably use them in your builds.

Deploy Artifact Bundles (multiple artifacts in one go)

in future plans

Import local repositories

Import repositories and separate RELASE and SNAPSHOT artifacts

Releases and Snapshots should be kept in separate repositories. The import tools can separate these artifacts for you into discrete repositories.

via Jsecurity + ExtJs user console. Full role based with the ability to specify permissions based on the path of the artifact (group/artifact/version) using regex if desired.

Support Prevention of Redeploy

in future plans

Control over who can populate caches

Fully featured procurement support included in the pro version. This allows absolute control over the artifacts allowed through based on the artifact and user.

Support Protection of Sources / javadoc etc

Using Ant-like simple to understand patterns + OOTB templates for common include/excludes

Using the regex to control the paths, it is possible to secure separately any artifacts you want. Comes configured with targets to specify sources, which would allow you for example to have jars be downloaded anonymously but not the sources, even though they are sitting in the same repository.

ordered control of cascading though configured realms -- as many as you have installed.

Secured settings.xml passwords

functionality already available in Maven 2.1.0

Centrally-controlled encrypted password policy so admins do not have to rely on clients security policy. Auto-generated encrypted passwords for your settings.xml.
Overcomes Maven drawbacks - Maven decrypts the password to clear-text on the client, and keeps a clear-text master password on the filesystem.

(we added the functionality to Maven core - 2.1.0)

Database

Database available for querying

configurable by datasources

Can be queried, but can also use JCR or REST API

n/a - Lucene index and REST api provided for searching.

Can run without database

Use by default an embedded Derby DB

Can use file-system storage. Database usage is recommended for fully transactional behavior of metadata not extractable from the artifact itself.